1. Field of the Invention
The present invention relates to tuning fork-type crystal vibrators (also referred to as “tuning fork-type vibrators” hereinafter) having grooves in both principal surfaces of a pair of tuning fork arms, and in particular to subminiature tuning fork-type vibrators whose shock resistance is improved and whose crystal impedance (also referred to as “CI” hereinafter) is kept small.
2. Description of the Related Art
Tuning fork-type vibrators are widely used as signal sources for keeping time in wrist watches, and in recent years, they have come to be used as synchronization signal sources in portable electronic devices. Thus, as electronic devices are becoming smaller, there is a growing demand for smaller tuning fork-type vibrators.
FIG. 7A and FIG. 7B are diagrams illustrating a conventional example of a tuning fork-type vibrator. FIG. 7A is a diagrammatic view of a tuning fork-type vibrator and FIG. 7B is a cross-sectional view of the pair of tuning fork arms taken along M—M in FIG. 7A, indicating the electric field direction.
The tuning fork-type vibrator includes, as shown in FIG. 7A, a tuning fork-shaped crystal element 1. As for the crystallographic axes (XYZ) of the crystal element 1, the X-axis extends along the width direction, the Y-axis extends along the length direction, and the Z-axis extends along the thickness direction. The tuning fork-shaped crystal element 1 is made of a tuning fork base portion 2 and a pair of tuning fork arms 3a and 3b. The leftward direction of the tuning fork-shaped crystal element 1 is the −X axis and the rightward direction is the +X axis. There is basically no directionality to the Y-axis and the Z-axis. There are grooves 4 in the two principal surfaces of each of the pair of tuning fork arms 3. The tuning fork base portion 2 protrudes in a sloping curve from the outer lateral surfaces of the pair of tuning fork arms 3a and 3b, and has two-level base protrusion portions 2a provided with a level step, widening the width of the tuning fork base portion 2. The curved slopes of the base protrusion portions 2a are formed in correspondence with a U-shaped tuning fork slit, so as to maintain the symmetry of the tuning fork arms 3a and 3b. 
Excitation electrodes 5 for exciting tuning fork vibrations based on bending vibrations are formed in the grooves 4 provided in the two principal surfaces of each of the pair of tuning fork arms 3 as well as on the two lateral surfaces of each of the pair of tuning fork arms 3 (see FIG. 7B). The excitation electrodes 5 respectively put the two principal sides and the two lateral surfaces of each of the tuning fork arms 3a and 3b to the same potential, and put the two principal surfaces at opposite potentials to the two lateral surfaces. Moreover, they are wired such that the two principal sides and the two lateral surfaces of the tuning fork arm 3a are at opposite potentials to the two principal sides and the two lateral surfaces of the tuning fork arms 3b, respectively.
As shown in FIG. 8A, one principal surface at the bottom portion of the tuning fork base portion 2 at which lead electrodes (not shown in the drawings) extend from the excitation electrodes 5 is fixed to a raised base 8 in a container 7 by an electrically conductive adhesive 6 or the like that is applied to a predetermined region (surface area) P″, thus holding the tuning fork-type vibrator while establishing electrical and mechanical connection. On the outer surface of the container 7, as shown in FIG. 8B, there are mounting terminals (not shown in the drawings) for surface mounting and the tuning fork-shaped crystal element 1 is sealed into the container 7 by placing a cover 9 onto the container 7. It should be noted that FIG. 8A is a front view of the tuning fork-shaped crystal element 1, and FIG. 8B is a cross-sectional view of the tuning fork-type vibrator.
Ordinarily, a plurality of tuning fork-shaped crystal elements 1 are formed by etching using photolithography. For example, a plurality of tuning fork masks 11 are formed by photolithography on a Z-cut crystal wafer 10 (see FIG. 9). Then, the crystal wafer 10 is placed into an etching solution of hydrofluoric acid or the like, and etched, thus obtaining a plurality of tuning fork-shaped crystal elements 1 processed to their outer shapes. In this case, after the processing of the outer shape of the tuning fork-shaped crystal elements 1, the grooves 4 are formed by etching of the two principal surfaces of the tuning fork arms 3a and 3b, and then they are separated into individual tuning fork-shaped crystal elements 1 (see FIG. 7A).
Thus, the grooves 4 are provided and the excitation electrodes 5 are formed in the two principal surfaces of each of the tuning fork arms 3a and 3b, so that the electric field (indicated by arrows in FIG. 7B) generated between the two principal surfaces and the two lateral surfaces of each of the tuning fork arms 3a and 3b is linear and parallel to the X direction. Consequently, compared to the case that the grooves 4 are not formed, the strength of the electric field in X direction can be increased, and strong bending vibrations in Y direction, caused by expansion and contraction in opposite directions at the two lateral surfaces of each of the tuning fork arms 3a and 3b, can be excited.
Moreover, due to the bending vibrations in Y direction, which are in opposite directions in the tuning fork arms 3a and 3b, the pair of tuning fork arms 3a and 3b performs a tuning fork vibration in an opening and closing motion in horizontal direction with the tuning fork slit portion as the center. Consequently, even when the tuning fork-shaped crystal element 1 is small, it is basically possible to obtain a tuning fork-type vibrator with a good CI value, due to the linear electric field that is parallel to the X axis in the tuning fork arms 3a and 3b. 
Moreover, the width of the tuning fork base portion 2 is increased beyond the outer lateral surfaces of the pair of tuning fork arms 3a and 3b by providing it with base protrusion portions 2a, so that it can be held by applying the electrically conductive adhesive 6 over a sufficient surface area P″ (see FIG. 8A and FIG. 8B). Consequently, damage when the tuning fork-type vibrator is dropped or impacted as well as breaking off of the tuning fork-shaped crystal element 1 can be prevented, thus achieving good mechanical shock resistance. If the width of the tuning fork base portion 2 is made the same as the width between the outer lateral surfaces of the pair of tuning fork arms 3a and 3b for example, then the surface area of the tuning fork base portion 2 becomes small, and also the amount of the electrically conductive adhesive 6 becomes small, thus worsening the mechanical shock resistance. Accordingly, if it is then attempted to provide the electrically conductive adhesive 6 with a large application surface area P, the electrically conductive adhesive 6 must be applied all the way to the bottom portion of the tuning fork arms 3a and 3b, which impedes the tuning fork vibrations and increases the CI value (see PCT International Publication WO 00/44092).
However, in the tuning fork-type vibrator with the above-described configuration, even though the tuning fork-shaped crystal element 1 can be made small, and even though a CI value specified to less than 90 kΩ can be attained by providing the pair of tuning fork arms 3a and 3b with the grooves 4 and a high mechanical strength (mechanical shock resistance) preventing damage or breaking off of the tuning fork-shaped crystal element through drops or impacts can be achieved by ensuring a large application surface area of the electrically conductive resin, there was the problem that it was not possible to attain an even smaller CI value or, in particular, better frequency change characteristics (electrical shock resistance) with regard to drops or impacts.
When the surface area over which the electrically conductive adhesive 6 is applied is large, then the adhesive strength is high and the mechanical shock resistance is improved, but the influence that this manner of support has on the tuning fork vibrations becomes large, and the electrical shock resistance, such as frequency changes in response to drops or impacts becomes poor. Conversely, when the surface area over which the electrically conductive adhesive 6 is applied is small, then the adhesive strength is weakened and the mechanical shock resistance becomes poor, but the influence that this manner of support has on the tuning fork vibrations becomes small, and the electrical shock resistance is improved.
When a crystal wafer is processed by etching, then the etching speed differs depending on the axis direction of the crystal, a phenomenon that is known as “etching anisotropy.” The etching speeds in the crystal axis directions X, Y and Z are Z>>+X>−X>Y. Consequently, when a plurality of tuning fork-shaped crystal elements 1 are formed on a crystal wafer 10 as described above (see FIG. 9), then the etching anisotropy causes angular protrusions 12 to be formed on the +X surfaces on the lateral surfaces of the tuning fork-shaped crystal element 1, as shown in the partial front view of FIG. 10A and the top view of FIG. 10B. Also, in the U-shaped portion of the tuning fork slit and the curved portions of the base protrusion portions 2a, angular first and second slanted portions 13a and 13b that rise upward from the −X direction to the +X direction are formed.
Here, the first slanted portion 13a formed in the tuning fork slit is simultaneously etched into the +X surface and −X surface of the tuning fork arms 3a and 3b and the Y surface of the slit bottom. However, the differences of the etching speed can be caused in the +X and the −X directions so that it rises in the +X direction starting from a base point on the left side of the center line A—A bisecting the tuning fork-shaped crystal element 1 in the width direction. Consequently, there is a geometrical asymmetry with respect to the center line A—A bisecting the tuning fork-shaped crystal element 1 in the width direction, and the mass of the right half is larger than the mass of the left half.
Thus, due to the asymmetry (unbalance) in the masses of the two tuning fork arms 3a and 3b including the tuning fork slit, the center of the tuning fork vibrations within the tuning fork slit is displaced from the center line A—A to the right, that is, in the +X direction. As a result, the center of the tuning fork vibrations is shifted from the geometrical center on the center line A—A bisecting the tuning fork-shaped crystal element 1 and the balance is lost, so that vibrations are leaked from the tuning fork base portion 2, and the CI value increases.
In particular, when the length of the pair of tuning fork arms 3a and 3b is large, a shift of the center of the tuning fork vibrations from the geometrical center has a large adverse effect on the balance of the tuning fork vibrations. In this case, the tuning fork-shaped crystal element 1 becomes smaller while the interval between the two tuning fork arms 3a and 3b becomes narrower. Furthermore, the etching of the tuning fork slit is restricted. Thus the remaining mass of the first slanted portion 13a becomes large. Consequently, the influence that the asymmetry has on the CI increases.
Moreover, when the tuning fork-type vibrator is dropped or impacted, a stress depending on the asymmetry of the tuning fork-shaped crystal element 1 occurs, so that the adhesion with the electrically conductive adhesive 6 may change. Therefore, the manner of support of the tuning fork-shaped crystal element 1 is affected, and the vibration frequency may change. Consequently, there was the problem that the frequency change characteristics with regard to drops or impacts are worsened, so that the electrical shock resistance cannot be improved. Needless to say, the mechanical shock resistance, such as resistance against breakage in case of drops or impacts, is also worsened.
In these cases, the CI value of the tuning fork-type crystal vibrator can be made to satisfy the specified CI value for example by increasing the depth of the grooves 4, but it was not possible to prevent a change for the worse, in particular, the frequency change characteristics with regard to drops or impacts (electrical shock resistance). Thus, there was the serious problem of how to improve the electrical shock resistance in order to realize a small tuning fork-type crystal vibrator in which the planar outer dimensions of the tuning fork-shaped crystal element are not larger than 2.3 mm×0.5 mm, for example.
It should be noted that the base protrusion portions 2a may have the affect of reducing the asymmetry due to the first slanted portion 13a, but there was the problem that this alone is insufficient.
It is thus an object of the present invention to provide a tuning fork-type vibrator with decreased CI value and, in particular, with improved shock resistance.